stream: add final method

Adds the ability to for write streams to have an _final method which acts
similarly to the _flush method that transform streams have but is called before
the finish event is emitted and if asynchronous delays the stream from
finishing.  The `final` option may also be passed in order to set it.

PR-URL: https://github.com/nodejs/node/pull/12828


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>